1. Field of the Invention
The present invention relates generally to the field of chemical mechanical polishing of wafers, and more particularly to an improved polishing head with a floating knife-edge.
2. Description of the Prior Art
In the process of fabricating integrated circuits, it is essential to form multi-level material layers and structures on a wafer or die. However, the prior formations often leave the top surface topography of an in-process wafer highly irregular. Such irregularities cause problems when forming the next layer over a previously-formed integrated circuit structure. For example, when printing a photolithographic pattern having small geometries over previously-formed layers, a very shallow depth of focus is required. Therefore, there is a need to periodically planarize the wafer surface.
One technique for planarizing the surface of a wafer is chemical mechanical polishing (CMP). In CMP processing, a wafer is placed face down on a rotating platen. The wafer, held in place by a carrier or polishing head, independently rotates about its own axis on the platen. Typically, the head is a floating polishing head with a flexible membrane. On the surface of the platen is a polishing pad over which there is dispensed a layer of polishing slurry. The slurry chemistry is essential to proper polishing. Typically, it consists of a colloidal solution of silica particles in a carrier solution.
The floating polishing head generally provides a controllable pressure on the wafer backside to push the wafer against the polishing pad. As mentioned, some polishing heads include a flexible membrane that provides a mounting surface for the wafer, and a retaining ring to hold the wafer beneath the mounting surface. The retaining ring may be made of various hard polymer materials and is mounted on a base of the polishing head. Pressurization or evacuation of a chamber behind the flexible membrane controls the load on the wafer.
A problem encountered in CMP is the difficulty of removing the wafer from the polishing pad surface once polishing has been completed. When the wafer is placed in contact with the polishing pad with a layer of slurry on its surface, the surface tension of the slurry generates an adhesive force that binds the wafer to the polishing pad. Typically, the wafer is vacuum-chucked to the underside of the polishing head, and the polishing head is used to remove the wafer from the polishing pad. When the polishing head is retracted from the polishing pad, the wafer is lifted off the pad. However, if the surface tension holding the wafer on the polishing pad is greater than the vacuum-chucked force holding the wafer on the polishing head, then the wafer will remain on the polishing pad when the polishing head retracts. This may cause the wafer to fracture.
To solve the above-mentioned problem, a downwardly-projecting lip structure (also referred to as a xe2x80x9cknife-edgexe2x80x9d) fixed along the outer edge of a disk-shaped supporting plate is typically provided in some polishing head design. One such case is, for example, Titan Head(trademark), which is designed for Applied Materialsxe2x80x9d Mirra CMP system. However, the prior art fixed lip structure leads to another recurring problem in CMP, which is the so-called xe2x80x9cedge effectxe2x80x9d or xe2x80x9cfast-band effectxe2x80x9d, i.e., the tendency of the wafer perimeter to be polished at a faster rate than the wafer center, which results in poor intra-wafer uniformity. The fixed knife-edge structure helps to lift the wafer when the wafer polishing is completed, but planarity and uniformity suffers since the downwardly-projecting fixed knife-edge provides a larger downward force along the perimeter of the wafer than within the center region during the wafer polishing stage.
In one aspect, the invention is directed to a polishing head for a chemical mechanical polishing system. The polishing head includes a base, a retaining ring secured to the base defining a pocket area beneath the base, and a lower assembly floating within the pocket area by way of a diaphragm seal. The lower assembly includes a disk-shaped support plate having a plurality of apertures distributed in a center region of the support plate, a clamp ring used to secure the diaphragm seal along a rim region of the support plate, and a floating knife-edge mechanism positioned between the rim region and the center region of the support plate.
The support plate presents a substantially flat bottom surface for pressing a backside of a wafer, and provides a uniform downward force across the backside of the wafer during a CMP operation. When wafer polishing is completed, the floating knife-edge mechanism provides a downwardly-projecting lip portion to engage with the wafer so as to form a seal for improved vacuum-chucking. In another aspect, the present invention is directed to a lower assembly of a polishing head for CMP applications. The lower assembly comprises a support plate having a plurality of apertures distributed in a center region of the support plate, a clamp ring used to secure a diaphragm seal along a rim region of the support plate, and a floating knife-edge mechanism embedded in the support plate between the rim region and the center region. The floating knife-edge mechanism provides a substantially flat support plate bottom surface for pressing a backside of a wafer, and a uniform downward force across the backside of the wafer during a CMP operation.
According to one preferred embodiment of the present invention, the floating knife-edge mechanism comprises a discontinuous upper portion and a continuous annular lower portion. The discontinuous upper portion of the floating knife-edge mechanism is pushed downwardly by an independent bladder to engage with the wafer when a CMP operation is completed.
Advantages of the invention include reliable removal of a wafer from a polishing pad, minimal fast-band effects, and improved flatness and uniformity of the wafer.
Other advantages and features of the invention will be apparent from the following description, including the drawings and claims.